In the data signal transmission through telephone channels and in particular for two-wire lines, the socalled semi-duplex operation is carried out, in which between two subscriber points alternately data signal blocks and acknowledgement signals are transmitted, in order to keep as low as possible, for example, the load of the transmitting devices and also the lines. A grounded data signal block contains for example 100 signals within an entire duration of, for example, 120 msec. at a transmission speed of 4800 Bd. In carrying out the semi-duplex operation, however, a time is required for the alternate switching of the data signal blocks and the acknowledgement signals, which many times exceeds the time which is required for transmitting such a data signal block.
In view of these conditions, it is required, in the semi-duplex operation, that the switching over between transmitting and receiving functions of the transmitting devices be as short as possible, in order to have available an optimum amount of time for the actual data transmission.
Data signal and receiving apparatus connected to transmitting lines contain a variable-gain amplifier, which amplifies arriving data signals within a specified level zone always to such a level and balances out level variations, that they can be evaluated perfectly. However, each variable-gain amplifier has a control time constant, which stands against the above-illustrated requirement of switching-over times which are supposed to be as short as possible. If, for example, a transmitting device is switched over from transmitting action to receiving action, prior to the evaluation of arriving data signals first the control time constant must expire, after which time an evaluatable data level is available. If, additionally in the sense of a transmission speed which is as high as possible at a specified band width, combined modulation types, for example, a phase amplitude modulation, are applied, in order to hold the signal-interference relationship as high as possible, the disadvantages of such a function become particularly visible.
Therefore, the purpose of the invention consists in improving a circuit arrangement of the above-mentioned type so that times which are as short as possible for the creation of an evaluatable data level at switching-over operations between various types of operation are available. For this, a control circuit is produced which meets, on one side during signal transmission, all existing control requirements and, on the other side during a transmission interruption and the arrival of new data signals, delivers practically immediately an evaluatable signal level.
A first attainment of the foregoing purpose consists inventively in a switch controllable depending from the level of the arriving data signals being arranged in front of a buffer store in the connection between an integral control and a correcting element, and in the output signals of the buffer store and the output signals of a control controlling a difference amplifier, which delivers a control signal holding the control at a predetermined control condition.
With this circuit arrangement it is easily possible, to meet the aforediscussed requirements. If the switch is closed in the connection between the integral control and the correcting element at the presence of a sufficient signal level, then a normal control loop is realized, within which the integral control delivers a control signal to the correcting element, so that same within specified limits holds constant the level of the received data signals. If, however, the data signal level drops below a predetermined value, for which a switching threshold can be determined, the switch is opened and the buffer store arranged after the switch delivers the earlier existing value of the control signal for the time of the data interruption to the correcting element. The correcting element maintains the amplification on the data signal transmitting path at the earlier existing value, so that at newly arriving data signals, which at semi-duplex operation according to definition have the earlier existing level, the specified amplification exists immediately again. However, one must consider hereby that when a further switching measure is missing, the integral control would change, namely "run-up" its output signal during the interruption of the data signals because of the missing of a normal magnitude signal, so that when the switch is anew closed, a changing of the adjusting value would occur. For this purpose according to a further characteristic of the invention, a difference amplifier is provided, which is controlled on one side through the output signals of the buffer store, thus through the control signal delivered to the correcting element and on the other side through the output signals of the control. If between these two control signals a difference occurs, which is the case when the integral control runs-up, an output signal can be taken from the difference amplifier, which is utilized at the input of the integral control for the compensation of such a signal change.
A circuit arrangement according to the invention meets all requirements of the semi-duplex operation. The circuit assures quick switching-over times by storing the control signal and the control signal can be removed by the deriving of a signal, characterizing the signal level in the data signal transmitting devices, for the level reporting of the conventional type. Further, it can control a switch device which, through the on and off switching of the receiving channel in the respective device, exactly define on-off conditions so that misevaluations due to spurious signals are avoided.
A second attainment of the above-mentioned purpose lies for a circuit arrangement of the above-mentioned type in a switch which can be controlled depending on the level of the arriving data signals arranged in front of the integral control, and in the integral control including a field effect operational amplifier having a capacity storing the normal magnitude signal and being held at a fixed reference voltage at the input. An additional advantage of this attainment is that in particular individual operations and balancing operations are not needed during the manufacture. A buffer store and a difference amplifier is not needed because a direct control of the correcting element by the integral control is possible. A line-dependent adjustment is not required.